config: Apply formatting and remove unnecessary namespace documentation

[stotko]

The config.h.in file is a placeholder for the auto-generated config file and has so far been not considered to be formatted by clang-format. Since the special replacement syntax of CMake breaks the automatic formatting, perform a manual correction afterwards and keep the file still excluded.

While at it, also clean up the _fwd extension from the file detection and also remove the unnecessary documentation of the stdgpu namespace which generates a bloated page in the documentation that is not referenced anywhere.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (349cfaf) 97.23% compared to head (cfc9b1c) 97.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #357 +/- ## ======================================= Coverage 97.23% 97.23% ======================================= Files 31 31 Lines 2421 2421 ======================================= Hits 2354 2354 Misses 67 67 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko)

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